i) Field of the Invention
This invention relates to a process for producing a cement clinker having an effective content of blast furnace slag; more especially the invention is concerned with a process in which a blast furnace slag having a content of water is the source of the blast furnace slag content of the cement clinker and is introduced directly into the cement clinker during a finishing stage of its manufacture.
ii) Description of Prior Art
In a cement plant, cement clinker is created at elevated temperatures in a cement kiln from cement clinker raw ingredients which travel through the kiln from a feed end to a discharge end, while passing through different processing zones at elevated temperatures.
The resulting hot cement clinker which typically has a temperature of about 1300.degree. C. at the discharge end of the kiln, is fed into a cooler and travels as a bed of clinker from the cooler entry port to the cooler exit port on a cooler grate. Air is blown through the bed from jets disposed below the grate to cool the hot clinker. The clinker at the cooler entry port has a temperature of about 1300.degree. C. and the clinker at the cooler exit port has a temperature of about 120.degree. C.
The cooled cement clinker is ground to a desired fineness and may be employed as such in cement or may be admixed with blast furnace slag to produce a blended cement.
The use of blast furnace slag in admixture with a hydraulic cement such as Portland Cement, is well established.
Typically the blast furnace slag is subjected to drying operations at elevated temperatures to drive off water present in the slag. The Portland Cement and the dried slag are then intermixed and interground to a desired fineness or the dried slag is ground to a desired fineness and thereafter mixed with ground Portland Cement.
Blast furnace slag is a by-product from the production of iron in a blast furnace; silicon, calcium, aluminum, magnesium and oxygen are the major elemental components of the slag.
Blast furnace slags include air-cooled slag resulting from solidification of molten blast furnace slag under atmospheric conditions; granulated blast furnace slag, a glassy granular material formed when molten blast furnace slag is rapidly chilled as by immersion in water; and pelletized blast furnace slag produced by passing molten slag over a vibrating feed plate where it is expanded and cooled by water sprays, whence it passes onto a rotating drum from which it is dispatched into the air where it rapidly solidifies to spherical pellets.
In general the glass content of the slag determines the cementitious character, rapidly cooled slags have a higher glass content and are cementitious; slowly cooled slags are non-glassy and crystalline and thus do not have significant cementitious properties.
Blended hydraulic cements containing blast furnace slag include Portland Blast-Furnace Slag Cement Type IS, Slag Cement Type S and Slag-Modified Portland Cement Type I (SM) all of ASTM C 595 M; and Portland blast-furnace slag cement Type 105, and Slag-Modified Portland Cement Type 10 SM both of CSA-A362.
Blast furnace slags contain 3 to 20%, generally 5 to 15%, by weight, of water which must be removed prior to grinding or intergrinding with Portland Cement. The removal of the water content of blast furnace slags is typically carried out in a separate purpose built drying oven, and this drying oven and its operation at elevated temperature represents an additional cost in the manufacture of blended hydraulic cements containing blast furnace slag.